Light-driven mechanism switch from heterogeneous to homogeneous photoreduction for triphenyl phosphate transformation in the pyrite/oxalic acid system.

Organophosphate esters (OPEs) are emerging contaminants of concern due to their increasing environmental occurrence and toxicity. Pyrite (FeS2), as a highly reactive iron sulfide mineral, has shown great potential for removing organic pollutants. Here, we investigated triphenyl phosphate (TPhP) transformation by FeS2, focusing on the regulatory role of oxalic acid (OA). Kinetic analysis demonstrated that 0.1 g/L OA enhanced TPhP transformation by FeS2, doubling the rate constant relative to pure FeS2 in the dark and elevating the efficiency from 66.6% to > 96% under simulated solar irradiation. Mechanistically, in the dark, reduced sulfur species (S2-, S22-) on the FeS2 surface acted as electron donors to reduce Fe(III), thereby promoting the Fe(II)/Fe(III) cycle. OA not only reinforced this cycle but also alleviated the surface passivation, thus sustaining the reactivity of FeS2. In contrast, under light, the mechanism shifted to the photoreduction of homogeneous Fe(III)-OA complexes, with dominant reactive species switching from ·OH to ·CO2-, reflecting a light-induced transformation from heterogeneous interface to homogeneous solution. Although transformation products retained developmental toxicity, the acute toxicity of several intermediates was substantially reduced. These findings establish OA as an effective activator for enhancing TPhP transformation by FeS2 and advance mechanistic insights across dark/light conditions.